Transport for Wales 769's

Sponsor Post - registered members do not see these adverts; click here to register, or click here to log in
R

RailUK Forums

samuelmorris

Established Member
Joined
18 Jul 2013
Messages
4,937
Location
Brentwood, Essex
I thought these engines ran at a constant speed so there was always 750V for the DC bus?
There are a few electronic components between the generator and the DC bus of the unit. For a start, an engine-driven generator will produce AC that has to be rectified into DC first. As part of that process the voltage will also be normalised to the 750V required.
 

AM9

Established Member
Joined
13 May 2014
Messages
9,338
Location
St Albans
Use of higher than normal (in some case full) engine speeds when decelerating or when stopped. There aren't a lot of regular runs of 769s on youtube and of course I haven't ventured down to Wales during lockdown to see for myself, but there's definitely been something odd about the way they behave up until now.
That's interesting that the engines are running that fast on pulling in, especially when there's absolutely no traction demand, and the ER braking should be in use. However those engines do sound as if they aren't under load, so maybe it is an issue with the demand management between the engines.
From the few clips that I've watched, they do seem to have a lot less thrash than Sprinters when pulling away as they don't need to spin any torque converters fast to get the torque to pull away. The genset probably adjusts the field excitation to generate enough power for normal acceleration at as low engine revs as the mechanical power can be delivered. Also, as Bletchleyite says, they should run steadily at the point on their power curve that delivers what the generators demand. The engine doesn't get involved in the intermittent torque required at the wheels as that is managed by the traction electronics and motors as long as the bus can supply the current.
There certainly isn't the thrash that (say) a 150 has to get going in order to slosh enough hydraulic fluid around.
 

samuelmorris

Established Member
Joined
18 Jul 2013
Messages
4,937
Location
Brentwood, Essex
Yeah exactly - they couldn't be under load as where would the energy be going? There's just something odd about how they're being managed in respect of the traction power demand. Since there aren't many videos of them other than test runs, that may well be an issue that's been fixed by now, but it does highlight that there were definitely some quirks and/or bugs with the engine control process that could explain some of the reliability issues. On pulling away, they sound much more like I'd expect from a DEMU, i.e. as you say without being thrashed at full speed from the get go, but considering how glacially slow they accelerate, one does wonder whether they are actually getting all the power from the engines they should. I wasn't expecting them to be fast, but I also wasn't expecting them to be as slow as they are.
 

AM9

Established Member
Joined
13 May 2014
Messages
9,338
Location
St Albans
Yeah exactly - they couldn't be under load as where would the energy be going? There's just something odd about how they're being managed in respect of the traction power demand. Since there aren't many videos of them other than test runs, that may well be an issue that's been fixed by now, but it does highlight that there were definitely some quirks and/or bugs with the engine control process that could explain some of the reliability issues. On pulling away, they sound much more like I'd expect from a DEMU, i.e. as you say without being thrashed at full speed from the get go, but considering how glacially slow they accelerate, one does wonder whether they are actually getting all the power from the engines they should. I wasn't expecting them to be fast, but I also wasn't expecting them to be as slow as they are.
If the engines aren't under load, it shouldn't cause them to fail, - just increase their run 'times' a bit. It sounds like the tandem control system might still be a problem.
On the acceleration front, I wouldn't be surprised if the drivers are still holding back a bit, especially as units are failing more than they might expect.
Are there any on-board clips of the 769s around Hengoed. The down line gradients vary either side of 1:100 in that area, so their acceleration shouldn't be as slow as the 1:50 departures from Heath.
 

southern442

Established Member
Joined
20 May 2013
Messages
1,862
Location
Surrey
Happens a lot. Reliability of 321s went through the floor when they moved to Northampton from Bletchley, for instance, though it picked back up again when they got used to them.
This makes sense. It seems to me, then, that the reliability issues being discussed here are analogous to teething troubles when new units first enter service, which means I don't totally understand why people are really laying into the 769s when this sort of to be expected.
 

samuelmorris

Established Member
Joined
18 Jul 2013
Messages
4,937
Location
Brentwood, Essex
If the engines aren't under load, it shouldn't cause them to fail, - just increase their run 'times' a bit. It sounds like the tandem control system might still be a problem.
On the acceleration front, I wouldn't be surprised if the drivers are still holding back a bit, especially as units are failing more than they might expect.
Are there any on-board clips of the 769s around Hengoed. The down line gradients vary either side of 1:100 in that area, so their acceleration shouldn't be as slow as the 1:50 departures from Heath.
I wasn't suggesting them being overworked would cause the engines themselves to fail, rather that this odd behaviour may be symptomatic of a control system that has a few issues yet to be ironed out and thus, could potentially be unreliable. I may be completely wrong about that, it's just a guess.
 

AM9

Established Member
Joined
13 May 2014
Messages
9,338
Location
St Albans
This makes sense. It seems to me, then, that the reliability issues being discussed here are analogous to teething troubles when new units first enter service, which means I don't totally understand why people are really laying into the 769s when this sort of to be expected.
The 769s (and the 319s before them) have been subject to "London cast-off" abuse here since 2015, and I wouldn't be surprised if some of the criticism still relates to that.

I wasn't suggesting them being overworked would cause the engines themselves to fail, rather that this odd behaviour may be symptomatic of a control system that has a few issues yet to be ironed out and thus, could potentially be unreliable. I may be completely wrong about that, it's just a guess.
It's a shame that there isn't any member of RUK who can share local knowledge of their problems. I think that we agree that there are continuing problems with the engine control system as southern442 says.
 

Richard Scott

Established Member
Joined
13 Dec 2018
Messages
2,345
There are a few electronic components between the generator and the DC bus of the unit. For a start, an engine-driven generator will produce AC that has to be rectified into DC first. As part of that process the voltage will also be normalised to the 750V required.
Yes, appreciate it's not a DC generator but point I was trying to make is that the engines run at a constant speed to ensure via the various pieces of equipment to ensure that the 750V DC bus is maintained at 750V DC.
 

samuelmorris

Established Member
Joined
18 Jul 2013
Messages
4,937
Location
Brentwood, Essex
Yes, appreciate it's not a DC generator but point I was trying to make is that the engines run at a constant speed to ensure via the various pieces of equipment to ensure that the 750V DC bus is maintained at 750V DC.
Not necessary though, engine speed to a generator strictly speaking determines the frequency, not the voltage - which becomes irrelevant as it's then rectified to DC. The voltage requirements can be met as long as the engine has the power to counter the resistance it encounters when load is drawn from the generator, and of course that the generator and rectification components are capable of producing the current being drawn. It's electromagnetism and it's complicated, but a constant engine speed is only actually required when producing AC power. To avoid wasting fuel, DEMUs are capable of running at lower engine speeds when not at full power, within the engine's acceptable parameters of course.
 

Richard Scott

Established Member
Joined
13 Dec 2018
Messages
2,345
Not necessary though, engine speed to a generator strictly speaking determines the frequency, not the voltage - which becomes irrelevant as it's then rectified to DC. The voltage requirements can be met as long as the engine has the power to counter the resistance it encounters when load is drawn from the generator, and of course that the generator and rectification components are capable of producing the current being drawn. It's electromagnetism and it's complicated, but a constant engine speed is only actually required when producing AC power. To avoid wasting fuel, DEMUs are capable of running at lower engine speeds when not at full power, within the engine's acceptable parameters of course.
I'm well aware of all of that but don't forget as soon as you take power you may end up with a significant voltage drop well below that of the 750V, fairly sure the engines on these run at constant speed whether electrical demand is low or high.
 

samuelmorris

Established Member
Joined
18 Jul 2013
Messages
4,937
Location
Brentwood, Essex
I've no idea, it's not obvious to me based on what I've seen so far. I expect voltage drop will occur to a certain degree, after all it certainly does from the transformer of regular EMUs, but that isn't necessarily engine speed related. Engines running at constant speed based on demand is definitely not the case with these, or with many other DEMUs. Take a look at the video I linked on the previous page and jump to 6m40. You'll notice three discernable engine speeds - about 1050rpm at idle, about 1500rpm as it pulls away, then increasing to 1800rpm around the 7m00 mark. That, to be fair, is normal behaviour that I would expect. It is the engine speeds being high upon arrival that I don't understand.
 

Richard Scott

Established Member
Joined
13 Dec 2018
Messages
2,345
I've no idea, it's not obvious to me based on what I've seen so far. I expect voltage drop will occur to a certain degree, after all it certainly does from the transformer of regular EMUs, but that isn't necessarily engine speed related. Engines running at constant speed based on demand is definitely not the case with these, or with many other DEMUs. Take a look at the video I linked on the previous page and jump to 6m40. You'll notice three discernable engine speeds - about 1050rpm at idle, about 1500rpm as it pulls away, then increasing to 1800rpm around the 7m00 mark.
Ok, I've not watched the video but 1050rpm sounds like a very fast idle. Other DEMUs don't have the issue of maintaining a constant voltage at the bus, was going on what I read and hoping someone had first hand experience of these units and was able to confirm or disprove what I was saying. Sounds like engines don't run at constant speed then but maybe still idle faster than most.
 

samuelmorris

Established Member
Joined
18 Jul 2013
Messages
4,937
Location
Brentwood, Essex
Ok, I've not watched the video but 1050rpm sounds like a very fast idle. Other DEMUs don't have the issue of maintaining a constant voltage at the bus, was going on what I read and hoping someone had first hand experience of these units and was able to confirm or disprove what I was saying. Sounds like engines don't run at constant speed then but maybe still idle faster than most.
it's not all that unusual for DEMUs, to be fair. Voyagers run around 1100 when in a 'ready' state but not under power, dropping down to 950-1000 ish when at stations.
 

squizzler

Established Member
Joined
4 Jan 2017
Messages
1,580
Location
Jersey, Channel Islands
Would the high engine speed whilst pulling up be some sort of “engine braking” whereby regenerated electricity is dissipated by spinning the engines rather than feeding into the more traditional resistor array?
 

Domh245

Established Member
Joined
6 Apr 2013
Messages
7,488
Location
nowhere
Would the high engine speed whilst pulling up be some sort of “engine braking” whereby regenerated electricity is dissipated by spinning the engines rather than feeding into the more traditional resistor array?

Possible, but wouldn't think it's a desirable thing to do particularly. I can't help but wonder if the engines are left high-revving in order to allow quicker power delivery in the event of the driver shutting off power and reapplying fairly rapidly (coasting or being signal checked for example), but this implies both incredibly sloppy control systems that don't have any visibility of speed and terrible response from the engines powering up from idle, neither of which I'm convinced would be true

It'd be interesting to compare these to the Alstom Régiolis which use the same engine, though some quick youtubing has failed to turn up any useful videos
 

Senna1210

Member
Joined
30 May 2019
Messages
67
No engine braking or regen, engine revs drop with speed below 5mph to idle conditions
above 5mph engine speeds increase to suit demand but have to have a raised "Traction idle" so that the engine power is avalible if the load onto the alternator increases which the system does not know untill the driver demands it so has to be able to supply the requested traction power

it is funy seeing people guess what the problems etc are without any basis,its a old fleet into a new depot with new to depot technology and a depot that is trying to support so may diferent classes of vehicle

At least Northern know the 319 side and how to keep that piece of the puzzle happy
 

samuelmorris

Established Member
Joined
18 Jul 2013
Messages
4,937
Location
Brentwood, Essex
It's not exactly without any basis. We don't profess to be experts, but it's definitely abnormal behaviour. Should 'traction idle' really be almost full speed?
 
Joined
17 Apr 2019
Messages
26
It's a shame that there isn't any member of RUK who can share local knowledge of their problems. I think that we agree that there are continuing problems with the engine control system as southern442 says.
I saw one fail at Pengam station. It looked like the brakes refused to come off as the train started. That said I am blissfully ignorant of the engineering on these units.

As a passenger I found the two I've ridden on most comfortable. Their definitely a nicer ride that a Sprinter.
 

Bikeman78

Established Member
Joined
26 Apr 2018
Messages
2,003
Possibly. But I wonder how well those 170s compared to the others when they were with their previous operator?
Anglia 170s MTIN MAA was 11,049 in January 2020 and 16,068 in January 2019 so already heading in the wrong direction.
 

AM9

Established Member
Joined
13 May 2014
Messages
9,338
Location
St Albans
Possible, but wouldn't think it's a desirable thing to do particularly. I can't help but wonder if the engines are left high-revving in order to allow quicker power delivery in the event of the driver shutting off power and reapplying fairly rapidly (coasting or being signal checked for example), but this implies both incredibly sloppy control systems that don't have any visibility of speed and terrible response from the engines powering up from idle, neither of which I'm convinced would be true

It'd be interesting to compare these to the Alstom Régiolis which use the same engine, though some quick youtubing has failed to turn up any useful videos
The Régiolis is a bi-mode train from new, and I presume has each diesel genset in the same car as the motors that it supplies. The difference with the 769 is that the train has a DC bus that is powered from transformer, 3rd rail or diesel drivern generator. As the motors are in a separate car to either of the gensets, it was necessary to feed the bus from both generaors at the same time. During development, there were problems of the two gensets hunting, i.e each one sensing that the other was providing enough power and taking a rest - and then both trying to meet the demand. This instability would be at its worst when the train was cruising as presumably a single engine alone could probably provide enough power for the traction system. The control system I believe is automatic in that it maintains the bus voltage - the driver's control acts on the traction electronics which expects the bus voltage to be mantained. It maybe that the load sharing between the two gensets needs further tweaking to prevent instability or unnecessary high tickover speeds.
 

Domh245

Established Member
Joined
6 Apr 2013
Messages
7,488
Location
nowhere
The Régiolis is a bi-mode train from new, and I presume has each diesel genset in the same car as the motors that it supplies. The difference with the 769 is that the train has a DC bus that is powered from transformer, 3rd rail or diesel drivern generator. As the motors are in a separate car to either of the gensets, it was necessary to feed the bus from both generaors at the same time. During development, there were problems of the two gensets hunting, i.e each one sensing that the other was providing enough power and taking a rest - and then both trying to meet the demand. This instability would be at its worst when the train was cruising as presumably a single engine alone could probably provide enough power for the traction system. The control system I believe is automatic in that it maintains the bus voltage - the driver's control acts on the traction electronics which expects the bus voltage to be mantained. It maybe that the load sharing between the two gensets needs further tweaking to prevent instability or unnecessary high tickover speeds.

It's a bimode from new, but it uses the same engines (and both are BM-DV although the second voltage on the régiolis is 1500v DC rather than 750) which is why it'd be interesting to see how they both compared, particularly with regards to things like idle speeds. Whilst they've got one engine per motor set, as far as I can tell these are mounted remotely to the traction equipment, so it may not be the case that they are one engine/motor (or accurately, axle!) but are bussed together á la 769s
 

hwl

Established Member
Joined
5 Feb 2012
Messages
6,326
It's not exactly without any basis. We don't profess to be experts, but it's definitely abnormal behaviour. Should 'traction idle' really be almost full speed?
The overall control system is roughly equivalent to Voyagers /Meridians i.e. without clever integration of engine and TCMS seen on the 80x (what TCMS on 769s!).

Voyagers /Meridians have a "traction ready" high idle 1/3 of the way up the rpm range and "traction" idle (or rather no power taken but traction engaged) at max engine speed.

Hence the 769 isn't really that different to the proven non rail genset type thinking used on the Voyagers /Meridians.

With most efficient genset set ups with modern electronic fuel injection and governing systems and 3 phase alternators, the normal running speed for the engine is ideally the maximum torque speed which allows good overall efficiency and quicker response to increases in power demand.

Thus for me the 769s set up is as expected given retrofitting into older "dumb" units.

High engine speeds with negligible loads after a period of high load can often be due to the need to run directly powered water pumps (and fans?) at faster than idle speeds to keep removing residual heat for a period (e.g. as the 66s do due to directly engine driven water pump).
 

AM9

Established Member
Joined
13 May 2014
Messages
9,338
Location
St Albans
It's a bimode from new, but it uses the same engines (and both are BM-DV although the second voltage on the régiolis is 1500v DC rather than 750) which is why it'd be interesting to see how they both compared, particularly with regards to things like idle speeds. Whilst they've got one engine per motor set, as far as I can tell these are mounted remotely to the traction equipment, so it may not be the case that they are one engine/motor (or accurately, axle!) but are bussed together á la 769s
So comparing the two, a Régiolis 'unit' seems to have two or more cars with a number of gensets each supplying a a traction system*, rather than the 769 case which has two generators feeding a DC bus from which the traction system gets its power.
* within a unit, there might be a service/hotel bus to enable the train to continue in service in the event of an individual genset/traction system failure.

So in diesel-electric mode, a three-car Régiolis unit would behave like a three DEMU consist. The 769 configuration requires more power than any available single underfloor genset can provide, so the solution was to use the inherited DC bus structure to parallel two smaller gensets that cauld be accommodated sub-floor. A simple concept other than the problem of two generators interacting with each other, each trying to maintain a steady voltage at their outputs with a load that varies between a few kW at standstill to over 500kW under acceleration, - something that was experienced during development testing and a (partial) fix designed in before production.

I started writing this post 90 minutes ago but was interrupted for a while. Since then, HWL's post has given a lot of useful info, especially wrt the 769's similarity to the Voyager/Meridian setup. Given the Voyager/Meridian's multiple configurations of powered cars and trailers, I assume that the larger engines and their co-location with motored bogies in the same car means that the traction power is not bussed, (especially as they do not operate in an EMU mode with a common feed). Therefore, a bus fed by multiple generators isn't an issue, so Brush had to create something that didn't allow them to compete with each other. Maybe HWL could comment.
 

Greybeard33

Established Member
Joined
18 Feb 2012
Messages
3,220
Location
Greater Manchester
It may be relevant that the air compressor on the 769 is inherited from the 319 and fed from the DC bus. During deceleration the compressor is likely to start up to recharge the main reservoir, as the driver modulates the brakes.

Therefore the engines need to be running fast enough to maintain 750V on the bus while the compressor is running.
 

Bikeman78

Established Member
Joined
26 Apr 2018
Messages
2,003
It may be relevant that the air compressor on the 769 is inherited from the 319 and fed from the DC bus. During deceleration the compressor is likely to start up to recharge the main reservoir, as the driver modulates the brakes.

Therefore the engines need to be running fast enough to maintain 750V on the bus while the compressor is running.
Thinking back to the class 205 DEMUs, there was no observable change in engine speed if the compressor cut in when the train was stationary. However, when the heaters were switched on, the engine did idle at a higher RPM.
 

big all

On Moderation
Joined
23 Sep 2018
Messages
861
Location
redhill
Thinking back to the class 205 DEMUs, there was no observable change in engine speed if the compressor cut in when the train was stationary. However, when the heaters were switched on, the engine did idle at a higher RPM.
No increase in speed when the compressor kicks in or indeed out, is because it operates at what ever the output of the engine is. Whereas the train heat requires minimum voltage to operate efficiently thus the revs being higher when in train heat only mode. When in traction and train heat mode the drivers power handle dictates the engine revs with power output being shared between traction and heating needs in an unregulated way. As its around 40 years since I trained on thumpers I cannot remember if train heat remains cut out below a certain power notch level but I don't think it does?

When you take power the engine returns to idle before the motor contactors close to give traction power and off you go :D
 

AM9

Established Member
Joined
13 May 2014
Messages
9,338
Location
St Albans
It may be relevant that the air compressor on the 769 is inherited from the 319 and fed from the DC bus. During deceleration the compressor is likely to start up to recharge the main reservoir, as the driver modulates the brakes.

Therefore the engines need to be running fast enough to maintain 750V on the bus while the compressor is running.
How much load does the compressor present? I have no idea but I would be surprised if it was more than 10kW, which would probably be deliverable from the alternator when the engines are running not much above tickover speed. Similarly, there is the saloon heating (20kW per 4 cars), intenal lighting (3kW per 4 cars), various electrical/electronic equipment (5kW) - all in all the total of all non-traction power I would expect to be no more than 50kW which is a small proportion of the 500+kW total capability, and probably deliverable from a relatively low input speed.
When the heavy demand of the traction system is needed, the alternator regulator will increase the stator current which will make the generator place a higher load on the engine shaft, requiring a greater fuel (and air) input to meet that load. Providing everything is within the normal operating range, the DC bus voltage will stay within its nominal range.

Thinking back to the class 205 DEMUs, there was no observable change in engine speed if the compressor cut in when the train was stationary. However, when the heaters were switched on, the engine did idle at a higher RPM.
I think that the Thumpers had DC generators and a 600V bus but apart from the lower low-speed performance of a DC generator, the principle is the same. However their engines did hunt when they were on tickover.
 
Last edited:

Senna1210

Member
Joined
30 May 2019
Messages
67
The compressor is powered from the MA set and is not tied into the Bus lines only the traction /Flex ties directly to the Bus

The fans are inverter driven from the flex side on the engine rafts and have no mechanical coupling
 

Top